Conventional poly(amide-acid) compositions derived from tetracarboxylic acid dianhydrides, such as pyromellitic dianhydride (PMDA) are often thermally converted to the desired polyimide compositions at very high temperatures, for example, in the neighborhood of 300.degree. C. While certain "chemical" imidization procedures are known to work at lower temperatures using, for example, acetic anhydride and triethylamine, such procedures also generate undesired isoimide structures. It would be desirable to be able to reduce the temperature of the imidization reaction without using chemical techniques which generate isoimide structures so that, preferably, shaped polyimide products can be formed at milder conditions.
Additionally, U.S. Pat. No. 4,837,300 to T. L. St. Clair et al. describes the need for more processable polyimide compositions and achieves such a result by incorporating the proper mix of flexibilizing units in the backbone of the linear aromatic polyimide compositions it describes. The imidization procedure used in this patent, however, is not described as being significantly lower than conventional especially in the case of tetracarboxylic acid dianhydride-derived poly(amide-acid) compositions such as those derived from PMDA.
F. W. Harris et al. in ACS Polymer Preprints Vol. 17, No. 2 (August 1976), pp. 353-358 review various structure-solubility relationships in aromatic polyimides and lists a variety of diamine-dianhydride combinations which were described in the literature as effective. Two recent reviews which discuss various attempts to achieve soluble aromatic polyimide compositions are: Polymers for High Technology, ACS Symposium Series 346, (1987), "Soluble Aromatic Polyimides for Film and Coating Applications" (Chapter 37) by A. K. St. Clair et al., pp. 437-444 and "High Heat Polyimides Are Easier To Process Now" by J. D. Summers et al., Modern Plastics International, March 1989, pp. 47-52. A recent patent dealing with highly soluble aromatic polyimides is U.S. Pat. No. 4,935,490 to R. A. Hayes.
Hergenrother et al. in J. Polymer Sci:Part A:Polymer Chemistry, Vol. 27, 1161-1174 (1989) discuss polyimides containing carbonyl and ether connecting groups and prepares several new polyimides including one prepared from 3,3', 4,4'-benzophenonetetracarboxylic dianhydride and bis(4-aminophenoxy-4-phenyl)phenyl phosphine oxide. Poly(amide-acid) films were placed on plate glass and were thermally cyclodehydrated, rather than being solution imidized.
British Patent No. 1,030,026 mentions polyamide-acid solutions which can be formed by reacting an aromatic diamine, such as bis[3,5-dimethyl-4-aminophenyl ether (1)]-p,p'-diphenylmethylphosphine oxide (see page 4, lines 1-2) with a tetracarboxylic acid dianhydride, such as pyromellitic acid dianhydride. This reference does not address the effect that a bis(aminophenoxyphenyl) phosphine oxide would have on the solution imidization characteristics or temperature of a tetracarboxylic acid anhydride, such as pyromellitic dianhydride.